The present disclosure relates to the subject matter contained in Japanese Patent Application No. 2001-264452 filed on Aug. 31, 2001, which are incorporated herein by reference in its entirety.
1. Field of the Invention
The present invention relates to an ink jet recording head and an ink jet recording apparatus, and particularly relates to an ink jet recording head for ejecting ink droplets from a plurality of ejectors arrayed in a matrix, and an ink jet recording apparatus mounted with the ink jet recording is head.
2. Description of the Related Art
Non-impact recording systems have features of high speed, high image quality, low noise, and so on, and prevail in current printers. Of them, ink jet printers which fly ink droplets from a plurality of nozzles so as to perform printing of characters, drawings, pictures, and the like, on recording paper, are in widespread use because the ink jet printers have features in small size, low cost and capability of performing photorealistic printing.
An ink jet recording head is designed as follows. That is, while the head is moved in the main-scanning direction, ink droplets are ejected selectively from a plurality of nozzles, for example, 24-300 nozzles per color, in accordance with an electric signal based on print data. Thus, the ink droplets are made to adhere to the surface of a medium to be recorded on, such as recording paper. Further, in combination of the operation to feed the recording medium in the sub-scanning direction perpendicular to the main-scanning direction, the recording head can print characters or drawings on the medium to be recorded on.
In the ink jet recording head configured thus, ink is stored in an ink pool provided to be shared by the plurality of nozzles. The ink in this ink pool is introduced into pressure chambers via narrow inlets provided in the nozzles respectively. Further, in each of the pressure chambers, pressure exerting on the ink is generated by a pressure generating unit such as a piezoelectric element actuated in response to the electric signal. Thus, an ink droplet is ejected from the nozzle. The ink droplet ejecting mechanism constituted by the nozzle, the pressure chamber, the inlet and the pressure generating unit will be referred to as xe2x80x9cejectorxe2x80x9d.
An example of an ink jet recording head configured thus is disclosed in JP-A-8-58089. FIGS. 16 and 17 are a sectional view and a plan view showing the ink jet recording head disclosed in the same publication respectively.
As shown in FIGS. 16 and 17, the ink jet recording head has a nozzle formation plate 61, an ink pool plate 61, a diaphragm formation plate 63 having ink supply diaphragms 63a (corresponding to the inlets), a sealing plate 64, a pressure chamber formation plate 65 and a pressure plate 66. These plates 61 to 66 are laminated in the order named. Each pressure generating unit is constituted by the pressure plate 66 and a piezoelectric element 67. A pressure wave (acoustic wave) is generated for the ink in a pressure chamber 71 by applying a voltage control signal between an upper electrode 68a and a lower electrode 68b. By the plates 61 to 66, an ink flow path is formed to reach-each nozzle 73 from the ink pool 69 through the ink supply diaphragm 63a, a communication-hole 70, the pressure chamber 71 and an ink communication hole 72.
In such an ink jet recording head, each ejector has the pressure generating unit constituted by the pressure plate 66 and the piezoelectric element 67, the nozzle 73, the pressure chamber 71 and the ink supply diagram 63a. Such ejectors are arrayed in a straight line as shown in FIG. 17, so as to form an ejector array 74. The ink jet recording head having ejectors arrayed in a straight line will be referred to as xe2x80x9clinear array headxe2x80x9d.
Such a linear array head using piezoelectric elements as pressure generating units had a problem in realization of high-density arrangement of ejectors due to characteristic limits of the pressure generating units and restrictions on the manufacturing technology. In order to align the ejectors in high density in the linear array head, it is necessary to reduce the pressure chamber width. It is therefore necessary to arrange the ink jet recording head out of elongated ejectors having a large aspect ratio.
However, when the pressure chamber width is reduced to achieve the high-density arrangement of the ejectors, the width of a movable area of the pressure plate is also reduced so that the bending rigidity of the pressure plate increases. Thus, a sufficient deformation amount of the pressure plate cannot be obtained. As a result, there arises a problem that it becomes difficult to eject a desired quantity of ink droplets. In addition, the pressure chambers can be formed by etching, machining, resin molding, or the like, but there is also a limit in the reduction of the pressure chamber width due to the accuracy limit of machining.
Thus, in the linear array head using pressure generating units each constituted by a pressure plate and a piezoelectric element, there was a limit in high-density arrangement, substantially about 120-180 pieces/inch, due to the performance limit of the pressure generating units and the restrictions on the manufacturing technology. In the linear array head, ejectors can be indeed arrayed zigzag for doubling nozzle density. In that case, however, there arises a new problem that the head size increases while the head cost doubles.
As an ink jet recording head to solve the foregoing problems, there is known a recording head in which a large number of ejectors each having a pressure chamber with an aspect ratio close to 1 are arrayed in a matrix so as to place nozzles in high density. Recording heads configured thus are disclosed in Japanese Patent No. 2806386, JP-A-9-156095 and Japanese Translations of PCT publication No.10-508808, respectively.
FIGS. 18 and 19 show the main portion configuration of the ink jet recording head disclosed in Japanese Patent No. 2806386. This recording head will be referred to below as xe2x80x9cmatrix array headxe2x80x9d because nozzles 75 are arrayed in a matrix.
The matrix array head has a nozzle plate 82, a distribution plate 83, a cavity plate 84 and a pressure plate 85. The plates 82 to 85 are laminated in the order named. The nozzle plate 82 has the nozzles 75. The distribution plate 83 has ink supply grooves 79 and ink passageways 77. The cavity plate 84 has pressure chambers 76 and branch paths 81. Piezoelectric elements 86 are fixed to the pressure plate 85.
In the matrix array head, as shown in FIG. 19, a plurality of ink supply grooves 79 (corresponding to the branch flow paths) communicating with a not shown ink supply source (corresponding to the main flow path) are formed in parallel with one another between adjacent nozzles 75 and ink passageways 77. Further, each communication hole 80 is coupled with a branch path 81 provided for each pressure chamber 76, so that an ink flow path is formed. In such a matrix array head, there is an advantage that the nozzle density in the sub-scanning direction can be increased without reducing the width of each of the pressure chambers 76.
To secure a sufficient acoustic capacitance in an ink pool is a very essential problem for the inkjet recording head.
In the ink jet recording head, by the propagation of a pressure wave applied to a certain pressure chamber, not only is an ink droplet ejected from a nozzle communicating with this pressure chamber, but so-called acoustic crosstalk is produced. The acoustic crosstalk is a phenomenon that the pressure wave is also propagated through an inlet to the ink pool communicating with the pressure chamber. When the pressure wave is propagated to an adjacent ejector through the ink pool, a bad influence may be given to the ejection condition of a nozzle other than a desired nozzle. When this influence is conspicuous, there arises a phenomenon that a small amount of ink is also ejected from the adjacent nozzle other than the nozzle which has to eject ink, In order to suppress such a bad influence of acoustic crosstalk on adjacent nozzles, it is important that the pressure wave propagated to the ink pool through the inlet is absorbed and attenuated in the ink pool so that the pressure wave is prevented from being propagated to the adjacent ejectors. It is therefore necessary to provide a sufficient acoustic capacitance in the ink pool.
In addition, in the case that the acoustic capacitance of the ink pool is insufficient, the quantity of ink supplied from the ink pool to the respective pressure chambers runs short when the ejection frequency of ink droplets is increased or when the number of nozzles to eject ink droplets concurrently is increased. Thus, a stable ejection state cannot be obtained.
FIGS. 20A to 20F schematically show the meniscus behavior, in a nozzle portion before and after the ejection of an ink droplet. A meniscus 45 having a flat form initially (FIG. 20A) moves toward the outside of the nozzle when the pressure generating chamber is compressed. Thus, an ink droplet 46 is ejected (FIG. 20B). By the ejection of the ink droplet, the ink quantity in the inside of the nozzle is reduced so that a concave meniscus 45 is formed (FIG. 20C). The concave meniscus 45 returns gradually to the nozzle aperture portion by the action of the surface tension of the ink (FIG. 20D). Then, after repeating oscillation such as a slight overshoot (FIG. 20E) and a slight concave form (FIG. 20D) of the meniscus surface, the meniscus 45 is restored to its original state before the ejection (FIG. 20F). Here, as shown in FIG. 20C, the retracting position of the meniscus surface with respect to the nozzle surface is defined as y.
FIG. 21 is a graph showing an example of the positional displacement of a meniscus immediately after ink ejection. The meniscus making a large retreat (y=xe2x88x9260 xcexcm) immediately after the ejection (t=0) returns to its initial position (y=0) while oscillating as shown in the graph. The meniscus return behavior after the ejection of an ink droplet is referred to as xe2x80x9crefillxe2x80x9d in this specification, and time (tr) for the meniscus to be restored to the nozzle aperture surface (y=0) for the first time after the ejection of the ink droplet is referred to as xe2x80x9crefill timexe2x80x9d. Of FIGS. 20A to 20F, the refill time (tr) exists between FIG. 20D and FIG. 20E.
In order to eject ink droplets continuously in a stable state, it is important that next ejection is initiated after the completion of refill. In addition, in order to eject ink droplets continuously in a stable state, it is important that the meniscus shape is always retained in a fixed state immediately before ejection of an ink droplet. For example, when next ejection is initiated in the meniscus state before the completion of refill as shown in FIG. 20C, the diameter of an ink droplet ejected may be extremely small, or normal ejection of an ink droplet may be impossible, or bubbles may be caught from the nozzle surface so as to disable the nozzle from ejecting an ink droplet.
When next ejection is initiated in the state in which the meniscus is overshooted after refill with ink as shown in FIG. 20E, the axisymmetry of the meniscus shape maybe destroyed easily. Thus, bubbles may be caught from the nozzle surface so as to block ejection. Thus, if the time tr or longer has not passed since an ink droplet was ejected, next ink droplet ejection cannot be performed stably. For this reason, to secure a sufficient acoustic capacitance in the ink pool so as to achieve high-speed ink supply is an important characteristic parameter for dominating the maximum ejection frequency (that is, recording speed) of the ink jet recording head. In addition, when the refill time is not fixed among the ejectors, stable and continuous ejection cannot be achieved. It is therefore extremely important to secure a sufficient acoustic capacitance in the ink pool so as to suppress the shortage of ink supply to there by prevent a difference in refill characteristic among the ejectors.
When the quantity of an ejected ink droplet is reduced, it is possible to shorten the refill time, that is, to suppress the shortage of ink supply. In that case, however, it is impossible to obtain a sufficient printing density. When the number of nozzles ejecting ink droplets concurrently is limited or when the frequency of ejection is lowered, it may be possible to prevent the shortage of ink supply on one hand, but it is impossible to obtain a sufficient printing speed on the other hand.
As described above, in the ink jet recording head, in order to prevent acoustic crosstalk and prevent the shortage of ink supply, it is extremely important to secure a sufficient acoustic capacitance in the ink pool. A linear array head in which a pressure buffer unit has been disposed in an ink pool or on an ink pool wall is disclosed in JP-A-59-98860, JP-A-9-141864, JP-A-1-308644, or the like.
JP-A-59-98860 discloses a linear array head in which a pressure pulse absorbing member for absorbing a pressure wave is provided in a common ink chamber (corresponding to the ink pool). The pressure absorbing member is constituted by capsules wrapped in a thin plastic film. Each of the capsules is filled with gas such as the air or water vapor. JP-A-9-141864 discloses a linear array head in which a pressure absorbing member made of foam resin or, the like has been provided in an ink pool. JP-A-1-308644 discloses a linear array head in which a pressure-volume transducer made of an organic material or an elastic material and having a rate of 0.01 mm3/atm or more has been provided in an ink pool or in a position adjacent to the ink pool.
Examples in which a part of the wall surface forming an ink pool is formed of an easily deformable buffer member are disclosed in JP-A-59-42964, JP-A-2000-33713, JP-A-9-314836, and so on. JP-A-59-42964 or JP-A-2000-33713 discloses a drop-on-demand type print head in which a part of the wall surface of an ink pool different from the nozzle surface is formed of a buffer member made of a flexible film material. JP-A-9-314836 discloses a laminate type ink jet recording head in which an elastically deformable area is formed in the inner surface of an ink pool. The elastically deformable area is formed not in the outer layer surface on the nozzle surface side but in the inside of an ejector. The elastically deformable area is implemented by a thin portion (recess portion) made of a metal material and provided on one surface forming the ink pool.
Each of the disclosed examples of buffer members or the like described above is a disclosed example concerning a xe2x80x9clinear array headxe2x80x9d in which a plurality of ejectors communicate with a single common wide ink pool. As shown in FIG. 17, in the linear array head, the ink pool 69 can be disposed in an area different from the ejector array 74. Accordingly, there is an advantage that the wide ink pool 69 can be disposed regardless of the nozzle density of the ejector array 74. Thus, in the linear array head, though there are problems in high-density arrangement of the ejectors as described previously, a sufficient capacitance can be secured in the ink pool easily by installation of a pressure wave absorbing member or the like.
In each of the disclosed examples of buffer members or the like, a damper mechanism such as a pressure relaxing unit or a thin portion is formed in the inside of each ejector. Thus, a special constituent member and a special working process are required for forming such a pressure damper. The configuration is complicated, and the working process is troublesome.
In a matrix array head, there is indeed an advantage that high density of nozzles can be achieved easily, but the head has to be formed of narrow branch flow paths. It is therefore difficult to realize a pressure damper having a sufficient capacitance. In addition, differently from a linear array head, there are a large number of pressure chambers communicating with the plurality of branch flow paths in the head. Therefore, when the pressure damper is disposed in the inside of each ejector including its pressure chamber as described above, the configuration is further complicated in comparison with the linear array head, and the working process becomes more troublesome. Thus, there arises such a problem that the manufacturing cost increases.
In consideration of such problems, it is an object of the invention to provide an ink jet recording head in which a high-density nozzle array is realized in a matrix array head, while a sufficient acoustic capacitance is secured in a plurality of branch flow paths in a simple configuration and at low cost so that acoustic crosstalk can be suppressed, the shortage of ink supply can be prevented, and high-speed ink refill operation can be achieved, and to provide an ink jet recording apparatus having such an ink jet recording head.
In order to attain the foregoing object, an ink jet recording head according to the invention including an ink supply port, a flow path to which ink is supplied from outside through the ink supply port, a plurality of ejectors communicating with the flow path, respectively, each of the plurality of ejectors including a pressure chamber communicating with the flow path, a pressure generating unit for generating a pressure wave in ink charged into the pressure chamber, and a nozzle for ejecting the ink from the pressure chamber due to the pressure wave, a nozzle plate in which the nozzles are formed, and a damper member covering the flow path for suppressing crosstalk occurring among the plurality of pressure chambers. The nozzle plate is used as the, damper member.
xe2x80x9cPressure damperxe2x80x9d described in this specification is a general term of any unit for absorbing a pressure wave or any extremely easily deformable member forming a part of a wall surface.
In the ink jet recording head according to the invention, while a matrix array head having a large number of pressure chambers communicating with a plurality of branch flow paths is used, a complicated configuration in which a pressure damper is disposed in the inside of each ejector including its pressure chamber is not necessary. Thus, the working process becomes so simple that reduction in cost can be expected. In addition, a sufficient acoustic capacitance can be secured in each branch flow path without adding any special constituent member or any special working process such as providing a special pressure absorbing unit, forming a recess portion or forming a thin portion. In this case, it is preferable that one surface of walls of each branch flow path is formed in the nozzle-side outer layer surface which will be an interface with the external air layer, and the branch flow path wall is formed of a damper member having a low Young""s modulus.
In addition, when the damper member is formed of a one-piece member shared by a plurality of branch flow paths, an ink jet recording head having a sufficient acoustic capacitance and capable of suppressing acoustic crosstalk sufficiently can be obtained with a low-cost and simple configuration provided for the plurality of branch flow paths.
Here, it is preferable that the damper member satisfies:
cp greater than 10cnxe2x80x83xe2x80x83(1)
where cp designates the acoustic capacitance of the branch flow path per ejector and cn designates the acoustic capacitance of the nozzle. Alternatively, instead of the expression (1), it is also preferable that the damper member satisfies:
xe2x80x83cp greater than 20ccxe2x80x83xe2x80x83(2)
where cp designates the acoustic capacitance of the branch flow path per ejector and cc designates the acoustic capacitance of the pressure chamber. In these cases, not only is it possible to suppress acoustic crosstalk, but it is also possible to supply a sufficient quantity of ink to the respective ejectors from the branch flow path at a high speed. Thus, all the ejectors can eject ink droplets concurrently and stably at a high frequency.
The xe2x80x9cacoustic capacitance cp of the branch flow path per ejectorxe2x80x9d according to the invention means a value obtained by dividing the acoustic capacitance of one branch flow path by the number of ejectors disposed to communicate with the branch flow path.
In the related art, the conditions of the acoustic capacitance of an ink pool in a linear array head to suppress acoustic crosstalk and to prevent the shortage of ink supply are disclosed in JP-A-56-75863 or JP-A-59-26269. JP-A-56-75863 (Related-Art Technique A) discloses that the volume of a common ink flow path is set to be twice or more times as large as the total sum of the volume of pressure generating chambers (including flow paths in the neighborhood) so that the occurrence of crosstalk can be suppressed. JP-A-59-26269 (Related-Art Technique B) discloses an ink jet recording head in which impedance ZR of a common ink flow path is set to satisfy the relation ZRxe2x89xa6ZS/(10N) on the basis of the number N of ejectors connected to the common ink flow path and impedance ZS of an ink supply path so as to suppress the occurrence of crosstalk. In such a manner, in the disclosed examples (Related-Art Techniques A and B), the capacitance or impedance of the common ink flow path was set on the basis of the capacitance of the pressure generating chambers or the impedance of the ink supply path. However, from the results of experiments made by the present inventors, which will be described below, it was proved that stable ink droplet ejection could not be achieved under such conditions.
The inventors have made lots of experimental ejection observation, fluid analysis, equivalent circuit analysis, and so on. As a result, it is found that the variation amount of refill time in accordance with the number of ejectors ejecting ink droplets concurrently is dominated by the ratio of cp to cn, and crosstalk is dominated by the ratio of cp to cc. That is, in the ink jet recording head according to the invention, the value of cp to cn and the value of cp to cc are set to satisfy the conditions shown in the expressions (1) and (2) respectively. Accordingly, even in a head having a plurality of narrow branch flow paths as in a matrix array head, acoustic crosstalk can be suppressed, and the shortage of ink supply can be prevented. Thus, ink droplets can be ejected from a large number of ejectors continuously, concurrently and stably (U.S. patent application Ser. No. 10/118,805). Description will be made below on how the inventors have developed the invention.
First, description will be made on how the inventors have found the conditions to prevent pressure wave interference among ejectors, that is, acoustic crosstalk. The inventors have made trial production and evaluation of a large number of heads, and acoustic analysis thereof using a head equivalent circuit shown in FIG. 13. As a result, the inventors have discovered that the rate of occurrence of acoustic crosstalk depend substantially only on the ratio of cp to cc. Here, the signs c, m and r in FIG. 13 designate acoustic capacitance, inertance and acoustic resistance respectively, and suffixes d, n, i, c and p designate a piezoelectric element, a nozzle, an inlet, a chamber and a branch flow path respectively. For example, cd designates, an acoustic capacitance of a piezoelectric element. Incidentally, analysis is made on the assumption that the wide main flow path had a sufficient acoustic capacitance.
With reference to the analysis of the equivalent circuit in FIG. 13, how the rate of occurrence of acoustic crosstalk changes in accordance with the change of cp/cc is examined. FIG. 14 shows the result thereof. Here, the rate of occurrence of acoustic crosstalk is defined as:
rate of occurrence of acoustic crosstalk=(v2xe2x88x92v1)/v1 on the basis of droplet velocity v1 when one ejector is driven to eject an ink droplet independently and droplet velocity v2 when all the ejectors are driven to eject ink droplets concurrently.
As shown in the graph of FIG. 14, the rate of occurrence of acoustic crosstalk increases gradually with the increase of the value cp/cc, increases suddenly near the point where the value cp/cc exceeds 0.1, and reaches a peak when the value cp/cc is 1-2. After that, the acoustic crosstalk decreases suddenly with the increase of cp/cc, and then it is understood that the rate of occurrence of acoustic crosstalk can be suppressed to 7-8% or less if the condition cp greater than 20cc is satisfied.
It is understood that the rate of occurrence of acoustic crosstalk can be more preferably suppressed to 5% or less if cp greater than 50cc, and to 1% or less if cp greater than 100cc. Acoustic crosstalk increases conspicuously when the value cp/cc is 1-2. The reason causing the increase can be considered as follows. That is, a pressure wave propagated from a pressure chamber brings about oscillation of a pressure wave in the ink in a branch flow path. Since the oscillation frequency of the pressure wave oscillation produced in the branch flow path is close to the oscillation frequency of the pressure wave oscillation in the pressure chamber, both the oscillations interfere with each other, causing a kind of resonance phenomenon.
Strictly, the inertance mp or the acoustic resistance rp of the branch flow path also has an influence on the rate of occurrence of acoustic crosstalk. In an ordinary ink jet recording head, however, it is found that the influence is extremely small so that the rate of occurrence of acoustic crosstalk is substantially dominated by the value cp/cc as described above. The absolute value of the rate of occurrence of acoustic crosstalk varies in accordance with the head shape such as the nozzle shape, the inlet shape, or the pressure chamber shape. It is, however, confirmed that the correlation of increase/decrease of the rate of occurrence of acoustic crosstalk with the value cp/cc is constant regardless of the head shape as shown in FIG. 14.
In the same manner, the inventors carry out trial production and evaluation of heads, and analysis of their equivalent circuits. As a result, the inventors discover that the ink refill time depended on the ratio of cp to cn. FIG. 15 is a graph showing the result of an examined relationship between the value cp/cc and the refill time tr. From the graph, it is proved that the refill time is substantially constant regardless of the value cp/cn before the value cp/cn reaches 1, but the refill time increases suddenly when the value cp/cn exceeds 1, and then reaches a peak when the value cp/cn is 3-4. After that, the refill time decreases suddenly with the increase of cp/cn. Thus, it is made clear that the refill time can be prevented from increasing suddenly if the condition cp greater than 10cn is satisfied.
The reason why the refill time increases suddenly to reach a peak when the value cp/cn is 3-4 can be considered as follows. That is, a pressure wave in a pressure chamber interferes with a pressure wave in a branch flow path in the same manner as in the case of acoustic crosstalk. The absolute value of the refill time varies in accordance with the head shape such as the nozzle shape, the inlet shape, or the pressure chamber shape. It is, however, confirmed that the correlation of increase/decrease of the refill time with the value cp/cn is constant regardless of the head shape as shown in FIG. 15.
From the result of trial production of a plurality of kinds of ink jet recording heads, the following fact is made clear. That is, the influence of the inertance mp and the acoustic resistance rp of the branch flow path on the increase of the refill time are also small. Thus, in an ordinary ink jet recording head, it will go well if the properties of branch flow paths are set on the basis of the value cp/cn.
As described above, the inventors have found that in order to suppress acoustic crosstalk and the shortage of ink supply, it goes well if the two conditions of cp greater than 10cn and cp greater than 20cc are satisfied. In addition, it is also found that particularly with the setting in a range of 0.1 less than cp/cc less than 10 or 1 less than cp/cn less than 10, extremely great acoustic crosstalk occurs or the refill time increases suddenly. The ink jet recording head according to the invention has a feature in that the acoustic capacitance of the ink pool is optimally set to satisfy the two conditions of cp greater than 10cn and cp greater than 20cc on the basis of these results. When the conditions are satisfied, even in a matrix array head having narrow branch flow paths, it is possible to suppress the increase of refill time and suppress acoustic crosstalk.
In the ink jet recording head according to the invention, when the damper member is disposed on the nozzle outer layer surface side in a matrix array head, the damper member can be used also as the nozzle plate. As a result, nozzles can be formed directly in the damper member. With such a configuration, the number of parts and the number of manufacturing steps are reduced. Thus, even in a matrix array head having a plurality of branch flow paths, a pressure damper can be formed at low cost.
In the ink jet recording head according to the invention, it is preferable that the plate thickness of the damper member is not smaller than 20 xcexcm and not larger than 100 xcexcm. When nozzles are formed in the damper member, it is important to optimize the plate thickness of the damper member so that the pressure damper function and the nozzle function can be made compatible. When the plate thickness of the damper member is reduced, it is indeed possible to increase the acoustic capacitance of the ink pool. But it is proved that when the plate thickness is reduced excessively, there arose a problem that bubbles are apt to be caught from the nozzle surface when ink droplets are ejected.
The inventors investigate the relationship between the nozzle length and the catch of bubbles. As a result, it is experimentally confirmed that the nozzle length has to be 20 xcexcm or more in order to prevent bubbles from being caught. On the other hand, when the nozzle is extremely long, the inertance of the nozzle increases. Thus, there arises a problem that the efficiency in ejection becomes so low that the refill time increases. In addition, in an ordinary ink jet recording head, the nozzle diameter is about xcfx8630 xcexcm or less. However, to form such minute nozzles on a nozzle plate with high precision, there is a processing limit in the nozzle length. In order to satisfy these conditions, it was experimentally confirmed that the nozzle length had to be not larger than 100 xcexcm, preferably not larger than 75 xcexcm.
In the related-art matrix array heads, there is no description on specific implements for providing a pressure damper for a branch flow path. Japanese Patent No. 2806386 and Japanese Translation of PCT publication No. Hei.10-508808 (U.S. Pat. No. 5,757,400) disclose an ink jet head in which a nozzle plate formed a nozzle is used as a member covering a branch flow path. However, both references do not discloses that this member suppresses the cross talk in the branch flow path, at all.
In the ink jet recording head according to the invention, it is desired that the damper member is made of a film-like organic compound. Examples of such film-like organic compound may include acrylic resin, aramid resin, polyimide resin, aromatic-polyamide resin, polyester resin, polystyrene resin, nylon resin, and polyethylene resin.
Generally, metal materials such as stainless steel, glass, ceramics, organic compounds, etc. may be used as the head constituent members. It is, however, preferable that an organic compound having a small elastic coefficient (Young""s modulus) is used to achieve a satisfactory pressure damper function. In addition, in the ink jet recording head according to the invention, it is necessary to form nozzles in the damper member. When such a film-like organic compound is used, nozzles can be formed easily with high precision by excimer laser processing. The damper member can be indeed formed of a metal material or ceramic. But, when a metal material or ceramic whose Young""s modulus isone or two digits larger than that of such an organic compound is applied to a matrix array head having narrow branch flow paths, it is necessary to form the damper member to be extremely thin.
In this ink jet recording head in which the damper member can be arranged to be exposed on the nozzle outer layer surface side, unexpected excessive stress may act on the damper member due to jamming of the paper or the like. It is therefore practically difficult to use an extremely thin metal material as the damper member. On the other hand, when the damper member is formed of a film-like organic compound, the plate thickness of the damper member can be made several times as thick as that in the case of a metal material. Thus, there can be obtained an effect that the damper member is not broken by external force caused by paper jamming or the like.
When the film-like organic compound is made of polyimide resin, the polyimide resin has a high heat resistance temperature. Accordingly, when polyimide resin is used for the damper member, a heat process, for example, at 270xc2x0 C., can be used in any processes after the head is assembled. Generally, various bonding-processes are used for assembling ink jet recording heads. When polyimide resin is used for the damper member, various thermosetting adhesive agents or various thermoplastic adhesive agents may be used. For example, when polystyrene resin is used for the damper member, an epoxy-based adhesive agent having a setting temperature of 200xc2x0 C. cannot be used. In addition, polyimide resin is a chemically stable material, and has a feature of having a superior chemical resistance to ink. Further, polyimide resin also has a feature in that nozzles can be processed out of the resin with extremely high precision without any burr or the like by excimer laser. Incidentally, xe2x80x9cpolyimide resinxe2x80x9d described in this specification means a high polymer compound having an imide bond in its principal chain.
In a preferred ink jet recording head according to the invention, the pressure generating unit includes a piezoelectric element and a pressure plate for transmitting displacement of the piezoelectric element to the ink in the pressure chamber, and a maximum droplet quantity the pressure generating unit can eject is set to be not smaller than 15 pl (pico-liter). In this case, a large ink droplet of 15 pl or more can be ejected. Accordingly, a good image can be formed with printing resolution in a range of from 300 dpi to 600 dpi. In comparison with the case of printing with high resolution of 1,200 dpi, much higher speed printing can be achieved. In addition, it is preferable that the pressure generating unit having the piezoelectric element and the pressure plate is constituted by a piezoelectric actuator in which the pressure plate is flexibly deformed in accordance with extensible deformation of the piezoelectric element. In this case, a matrix array head can be realized easily
To print good characters or good images in an ink jet recording system, printing resolution of at least 300 dpi, preferably 600 dpi or higher, is required. From the fact that almost all of ink jet recording printers manufactured currently have resolution of 300 dpi or higher, it is understood that the resolution is an indispensable condition to secure image quality (excluding a draft print mode for high speed printing).
When printing is performed in the printing resolution of 300 dpi by use of water-based dye ink generally used, a maximum ejected droplet quantity of at least 15 pl, preferably 20 pl or more, is required for obtaining a sufficient image density without any color missing. Similarly, when printing is performed in the printing resolution of 600 dpi, a maximum ejected droplet quantity of at least 10 pl, preferably 15 pl or more, is required even by use of ink having a composition adjusted to extend its dot diameter on recording paper within a range not to degrade the image quality extremely. When the printing resolution is further enhanced, a required maximum droplet quantity is reduced. In this case, however, there arises a problem that the printing speed is lowered as will be described below. For example, when printing is performed in the resolution of 1,200 dpi, an image with sutficient density can be formed by a maximum droplet quantity of about 4-5 pl. However, when the printing resolution is improved, the printing data volume increases. Thus, when the number of nozzles is not changed, there arises a problem that the printing speed is reduced in accordance with the increase of the resolution. On the contrary, when the printing resolution is lowered to achieve high speed printing, there arises a problem that the image quality is degraded.
As a printing method to solve such conflicting problems and make the printing speed and the image quality compatible, there is known a droplet diameter modulation recording system in which the droplet quantity of an ejected liquid droplet is controlled. In the droplet diameter modulation recording system, a piezoelectric element is used as a pressure generating unit, and the waveform of a driving voltage to be applied to the piezoelectric element is controlled. Thus, the droplet diameter modulation recording system has a feature in that any droplet ranging from a small droplet having a small droplet quantity to a large droplet having a large droplet quantity can be ejected from one and the same nozzle. In combination of such a droplet diameter modulation technique, the image quality equivalent to that achieved by recording in high resolution of 1,200 dpi can be achieved in the printing resolution in a range of from 300 dpi to 600 dpi. However, even if the droplet diameter modulation technique is used, printing resolution is dominant over the character quality. For the character quality, the printing resolution of at least 300 dpi, preferably 600 dpi is required.
This ink jet recording head achieves the compatibility of the printing speed and the image quality with each other as described above. In addition, in order to adopt the droplet diameter modulation technique to achieve an excellent image and high speed printing in the resolution ranging from 300 dpi to 600 dpi, the ink jet recording head is configured as follows. That is, a piezoelectric element and a pressure plate for transmitting the displacement of the piezoelectric element to the ink in the pressure chamber are included as the pressure generating unit. In addition, a droplet quantity of at least 15 pl can be ejected. In this ink jet recording head, the pressure damper is designed so that even large ink droplets of 15 pl can be ejected continuously and stably at a high ejection frequency.
Pressure generating units using piezoelectric elements are roughly classified into a single-layer piezoelectric actuator and a multi-layer piezoelectric actuator. The single-layer piezoelectric actuator uses the flexible deformation of an actuator constituted by a piezoelectric element and a pressure plate, as its output. On the other hand, the multi-layer piezoelectric actuator uses the extensible deformation of a piezoelectric element made of a plurality of piezoelectric element layers laminated to one another, as its output. In a matrix array head having ejectors arrayed two-dimensionally, it is difficult to use such a multi-layer piezoelectric actuator from the point of view of the mounting technology and the manufacturing cost. It is preferable that an inexpensive single-layer piezoelectric actuator is used as the pressure generating unit.
Liquid ejected from nozzles is generically referred to as xe2x80x9cinkxe2x80x9d in this specification. Examples of such ink ejected from the nozzles in the ink jet recording head according to the invention may include printing ink, liquid containing an organic EL device material, or liquid containing an organic semiconductor material. When printing ink is used, the ink jet recording head can be applied to an ink jet recording apparatus which can obtain an excellent image. When liquid containing an organic EL device material is used, the ink jet recording head can be applied to an organic EL display manufacturing device, an organic EL display manufacturing head, and an organic EL display manufacturing apparatus, each using an organic EL display substrate as a target of application with the liquid. Further, when liquid containing an organic semiconductor material is used, the ink jet recording head can be applied to an organic semiconductor device manufacturing device, an organic semiconductor device manufacturing head, and an organic semiconductor device manufacturing apparatus, each using an organic semiconductor device substrate as a target of application with the liquid.